Pathologic outcomes of systemic therapy followed by stereotactic body radiation therapy (SBRT) for pancreatic cancer (PC) in a novel lateral decubitus treatment position.

Abstract

415 Background: Outcomes of multi-fraction stereotactic body radiotherapy (SBRT) for PC report low rates of toxicity and high local control, improving feasibility for combination with more aggressive systemic therapy regimens. However, SBRT in the ablative range poses risk to adjacent normal structures, excluding this option for tumors within 1 cm of a mucosal organ. In this study, we report our initial experience with treatment in the lateral decubitus position. Methods: An IRB retrospective query identified patients with pancreatic body adenocarcinoma treated with systemic chemotherapy followed by SBRT in the lateral decubitis position. SBRT was delivered to the entire gross disease with 30 Gy in 5 fractions with focal dose escalation up to 40 Gy to the tumor/vessel interface (TVI) as long as constraints were met. Patients were explored for resection if no metastasis or progression on restaging scans. The primary endpoints were pathologic response and margin status. Descriptive analysis was performed with SPSS 24. Results: The median age of the cohort was 68.6 (range, 50-83 yrs), with a white (94%) and male (59%) predominance. Initial staging of the 17 patients who met criteria included 10 (59%) patients diagnosed with borderline resectable disease (BRPC) and 7 (41%) with locally advanced disease (LAPC). 7 (41%) patients were treated with FOLFIRINOX, 5 (29%) with gemcitabine/nab-paclitaxel, 4 (24%) with gemcitabine/capecitabine/docetaxel, and 1 (6%) with gemcitabine/paclitaxel preceding SBRT. A median dose of 40 Gy (range, 33-40 Gy) was delivered to the TVI for all patients. 5 BRCP (29%) and 2 LAPC (12%) patients went to surgery, with 6 of these patients undergoing an R0 resection (86%) and 1 BRPC patient with an R1 resection. Pathologic tumor regression grades by the College of American Pathologists guidelines were 14% Grade 1, 71% Grade 2, and 14% Grade 3. Conclusions: Lateral decubitus treatment expands inclusion of pancreatic body patients for SBRT with focal TVI dose escalation leading to margin negative resection and significant partial tumor response, warranting future studies exploring ablative dosing in this position.